The invention relates to a transfer bump sheet for mounting bumps for flip chip interconnection to a semiconductor chip by transferring all bumps at a time to the semiconductor chip, a semiconductor flip chip having a semiconductor chip on which bumps for flip chip interconnection are formed, and a method of manufacturing the semiconductor flip chip.
In recent years, requirements for more functionality and miniaturization of electronic equipment have been increasing, and higher-density integration design and higher-density packaging design of electronic parts have been required accordingly. Therefore, the semiconductor packages used in such electronic equipment have been increasingly miniaturized and are provided with an increasingly large number of pins to a greater extent than in the past.
Conventional semiconductor packages which use lead frames have reached their limits on miniaturization and, therefore, new package methods of area array mount, such as BGA (Ball Grid Array) and CSP (Chip Scale Package), have recently been proposed as a package in which semiconductor chips are mounted on a substrate. In these semiconductor packages, wire bonding, TAB (Tape Automated Bonding), FC (Flip Chip) bonding, etc., are known as methods of electrical connection between pads of semiconductor chip and pads of substrate, which comprises one of various insulating material such as plastics and ceramics, and circuit trace, with a function of conventional lead frame. Recently, however, structures of BGA and CSP using FC bonding, which are favorable for the miniaturization of semiconductor packages, have frequently been proposed. In this FC bonding, bumps are usually formed beforehand on the pads of a semiconductor chip, the bumps being then located in correspondence to the terminals located on an interconnect substrate, and the bonding is performed by thermo-compression.
A vacuum evaporation method, printing method, solder ball aligning method, stud bump method, electrolytic plating method, and etc., are known as methods of directly forming bumps on a semiconductor chip (direct system).
In the vacuum evaporation method, Sn and Pb are evaporated through the use of a metal mask, and solder bumps are formed by means of a wet back. It is impossible to adopt this method in a case of narrow pitches because of the use of the metal mask. In addition, because the growth speed of bumps is low, it takes long time to form bumps for providing a required height, thus posing a problem.
In the printing method, solder-paste projections are formed by methods such as screen printing and are made to reflow, thereby forming solder bumps. This method is low in cost and has high productivity. However, in this method solder paste oozes during printing and sometimes comes into contact with the solder-paste projection formed on an adjoining pad. For this reason, it is difficult to adopt this method in a case of narrow pitches. On the other hand, when a printing method can be used in the case of narrow pitches, it becomes impossible to form bumps having a required height, thus posing a problem.
In the solder ball aligning method, ready-made solder balls are adsorbed with the aid of an adsorption tool provided with holes of the same alignment as the pad alignment of a semiconductor chip, which solder balls are then positioned on the pads of the semiconductor chip and are made to reflow, thereby forming solder bumps. The bump pitch depends on the solder ball diameter. Especially when solder balls are minute, dirt and dust on solder balls and the effect of static electricity sometimes induce failure in adsorption and the adhering of excessive solder balls or cause multiple solder balls to be adsorbed in a grape-like form by one hole of the adsorption tool. Thus, this method has the problem that solder bumps cannot be surely formed.
In the stud bump method, gold bumps or solder bumps are formed by bonding gold wires or solder wires to the pads of a semiconductor chip and cutting these wires. In this method, it is possible to cope with narrow pitches. When gold wires are used, gold bumps can be formed directly on the pads (aluminum pads) of a semiconductor chip and, therefore, there is an advantage that it is unnecessary to form barrier metal. In this method, however, because bumps are formed one by one on the pads of a semiconductor chip, the manufacturing time is long. In addition, the cost of manufacturing is high because the price of gold wire is high. Besides there is a fear that damage to a semiconductor chip may occur and, therefore, this poses the problem that this method cannot be adopted in a case of the area array mount.
In the electrolytic plating method, a plating mask is formed on a semiconductor chip, openings being formed in the positions of pads by exposure and development, and bumps are then formed by electrolytic plating. Because in this method bumps are formed to a required height by plating alone, the manufacturing time is long and the cost of the manufacturing is high. Further, because in the electrolytic plating method it is difficult to make the current density distribution in a plating bath completely uniform, variations in the height of formed bumps occur. The longer the plating time, the more remarkable variations in the height of the formed bumps become and, therefore, it is difficult to solve the problem insofar as the method of forming bumps by plating alone is concerned.
As mentioned above, the direct systems have various problems, and an improvement in the yield of bump forming is the greatest interest.
On the other hand, a transfer bump method has also been developed. In this method, an improvement in yield is aimed by transferring all bumps judged to have good quality to a semiconductor chip at a time. By locating a transfer bump sheet in place, in which bumps have been formed beforehand, in correspondence to a semiconductor chip and by performing heating and pressurization, all the bumps on the transfer bump sheet are transferred to the semiconductor chip at a time.
Methods of forming bumps on a transfer bump sheet, which have hitherto been known, are the vacuum evaporation method, printing method, bump punching method, electrolytic plating method, etching method, and etc.
The vacuum evaporation method has the problem that it is difficult to cope with narrow pitches and the problem that it takes time to form bumps as in the above vacuum evaporation method.
The printing method has the problem that it is difficult to cope with narrow pitches and the problem that bumps cannot be formed to a required height although the cost is low with high productivity similarly to the above printing method.
In the bump punching method, a metal ribbon is punched in the shape of bumps by means of a die and a punch and the bumps are aligned on a base sheet. Although this method has the advantage that the material for the metal ribbon can be freely selected, it has the problem that the manufacturing time is long because bumps are formed one by one on the base sheet. Further, although narrow pitches can be coped with by reducing the punch diameter, the service life of the punch becomes short because of its small diameter, thus posing a problem.
The electrolytic plating method has the problem that the manufacturing time is long and the cost of manufacturing is high and that variations in the height of bumps occur similarly to the above electrolytic plating method.
In the etching method, bumps are formed by etching a metal foil on a base sheet. Because bumps are formed by etching the metal foil, the manufacturing time can be shortened in comparison with the method of forming bumps by electrolytic plating. In addition, this method has the advantage that the bump height can be made uniform by making use of a metal foil with a uniform thickness. As the examples of the metal foil, there are known gold foil, solder foil and copper foil. In the case of using the gold foil, because bumps are made to remain in necessary portions by etching the gold foil, almost all of gold portions are removed by etching. This inevitably leads to a high cost of manufacturing. Further, it is very difficult to etch only the gold foil without eroding a base sheet. On the other hand, in the case of using a solder foil as metal foil, although solder foil is inexpensive in comparison with gold foil, it is still expensive in comparison with usual metal foil such as copper foil and besides procurement is limited. Further, in this case of the solder foil, the controlling of etching process is difficult, so that the manufacturing cost thereof can not help rising. In the case of the copper foil, the cost is low and the procurement thereof can be readily performed, however, there is such a problem as it can not be used as a bonding metal such as gold or solder.
On the other hand, in comparison with the direct system, a transfer system has such an advantage as only bumps which were judged to be good can be used. Thus, the most important problem of the transfer system is the yield of the bump transfer (the rate of transfer), and one of the important characteristics of the transfer system is adhesion occurring between a base sheet and bumps on the base sheet. Namely, the adhesion between the base sheet and the bumps is required to be, at the time of producing a transfer bump sheet, in a level at which the bumps surely adhere to the base sheet, and is required to be smaller than a bonding force occurring between a semiconductor chip after the bump transfer and bumps having been transferred. In conventional transfer bump sheets, the adhesion between the base sheet and the bumps was controlled to be within the above-explained range, however, this range was too narrow and was hard to be controlled, so that it was impossible to keep a sufficiently high transfer rate, or the yield of the transfer bump sheet was sacrificed.
In recent years, in order to ensure the reliability of bump bonding (for example, bonding strength after exposed to a high temperature), the use of bumps each having a metal core, such as copper-core solder bumps, instead of bumps made of solder alone has been proposed. In the ball aligning method, the reliability of bump bonding can be ensured by using ready-made copper-core solder balls. However, this leads to a substantial cost increase in comparison with the use of usual solder balls. Further, in the bump punching method, the metal core can be formed by using a metal ribbon in which both sides of copper foil are plated with solder. However, because of the problems in the bump punching method itself, it is difficult to adopt this measure. In the electrolytic plating method, it is possible to form copper-core solder bumps by reflow after the steps of solder plating, copper plating and solder plating. However, the cost of manufacturing increases because the manufacturing steps becomes more complex. On the other hand, in each of the vacuum evaporation method, printing method and stud bump method, it is impossible to form copper-core solder bumps.
The present invention was achieved as a result of intensive research performed to address the above problems in the structures and manufacturing methods of conventional transfer bump sheets.
Therefore, an object of the invention is to provide in a low cost and with a high yield a transfer bump sheet capable of transferring copper-core solder bumps with high bonding reliability to a semiconductor chip, which transfer bump sheet can transfer bumps with no substantial height variation at a time to a semiconductor chip and which transfer bump sheet has such reliability as no failure in transferring occurs (, that is, transferring ratio of 100% can be realized).
Another object of the invention is to provide in a low cost a semiconductor flip chip in which copper-core solder bumps with high reliability of bonding are mounted on a semiconductor chip through the use of this transfer bump sheet.
In the invention there is provided a transfer bump sheet for mounting bumps for flip chip interconnection to a semiconductor chip by transferring all of the bumps at a time to the semiconductor chip, comprising a base sheet for the transfer bump sheet, and metal posts located on the base sheet, each of the metal posts comprising at least a metal core and a solder coating, the metal core being formed by etching a metal foil, and the solder coating covering at least a part of surface of the metal core.
Also, in this invention there is provided a semiconductor flip chip having a semiconductor chip in which bumps for flip chip interconnection are formed, each of these bumps being provided with a metal core formed by etching a metal foil and a solder coating covering a part or whole surface of the metal core.
Also, in this invention there is provided a method of manufacturing a semiconductor flip chip, comprising the steps of preparing a transfer bump sheet according to the invention, locating the metal posts at positions corresponding to positions of pads provided in the semiconductor flip chip, and transferring all of the metal posts at a time by use of the transfer bump sheet.
In the transfer bump sheet of the invention, the metal posts comprises with at least a metal core formed by etching a metal foil, and a solder coating which covers a part or whole surface of the metal core. This metal post is preferably made of the two layers of a metal layer (metal core) and a first solder layer (solder coating) in the order from the base sheet of the transfer bump sheet. Alternatively, this metal post is preferably made of the three layers of a second solder layer (solder coating), a metal layer (metal core) and a first solder layer (solder coating) in this order from the base sheet.
The semiconductor flip chip of the invention preferably comprises a semiconductor chip in which bumps for flip chip interconnection are formed, each of the bumps comprises a metal core formed by etching a metal foil, and a solder coating covering a part or whole surface of the metal core. The material for the metal core is preferably copper.
The semiconductor flip chip of the invention comprises a semiconductor chip in which bumps for flip chip interconnection are formed. These bumps are formed by transferring all metal posts at a time through the use of the above transfer bump sheet.
The method of manufacturing a semiconductor flip chip of the invention comprises forming bumps for flip chip interconnection on a semiconductor chip by transferring in place all metal posts at a time in correspondence to the positions of the pads of the semiconductor chip by use of the above transfer bump sheet